The present invention relates to an ink supply amount adjustment method and apparatus for a printing press, which adjust the ink supply amount to a printing plate on the basis of the density value or color value of a printing product.
FIG. 16 shows the main part of the inking device (inker) in each color printing unit of a web offset printing press. Referring to FIG. 16, reference numeral 1 denotes an ink fountain; 2, ink stored in the ink fountain 1; 3, an ink fountain roller which forms part of the ink fountain 1; 4, a plurality of ink keys juxtaposed in the axial direction of the ink fountain roller 3; 5, an ink ductor roller which supplies the ink from the ink fountain roller 3 to ink rollers 6 by swinging between these rollers 3 and 6; and 8, a plate cylinder on which a printing plate 7 is mounted on its outer surface, and to which the ink is supplied from the ink rollers 6. An image is printed on the printing plate 7.
In this inking device, the ink 2 in the ink fountain 1 is supplied to the ink fountain roller 3 through the gap between the plurality of ink keys 4-1 to 4-n and the ink fountain roller 3. The ink supplied to the ink fountain roller 3 is supplied to the printing plate 7 through the ink rollers 6 by the duct operation of the ink ductor roller 5. The ink supplied to the printing plate 7 is transferred onto a printing paper sheet through a blanket cylinder (not shown).
FIG. 17 shows a printing product printed by this printing press. A band-shaped color bar 9-2 is printed on the margin portion of a printing product 9 except an image region 9-1 in the widthwise direction of the paper sheet. In general four-color printing, the color bar 9-2 includes a region S1 and regions S2 to Sn including density measurement patches (solid patches at a percent dot area of 100%) 9a1, 9a2, 9a3, and 9a4 of black, cyan, magenta, and yellow. The regions S1 to Sn correspond to the respective key zones of plurality of ink keys 4-1 to 4-n in each color printing unit of the printing press.
[Color Matching]
A reference density value is set in advance for each color (black, cyan, magenta, or yellow) printing unit. In printing the printing product 9, a color matching operation is done to make the density value of each color coincide with the set reference density value. This color matching operation is executed by the ink supply amount adjustment apparatus during final printing or before final printing on the basis of the density of a density measurement patch 9a (9a1, 9a2, 9a3, or 9a4) of each color in the color bar 9-2 printed on the printing product 9. “During final printing” means a period of printing printed matters as products. “Before final printing” means a period of preparation for printing when ink supply amount is adjusted while printing, prior to printing printed matters as products.
An ink supply amount adjustment operation will now be described by using the region S1 in the printing product 9 as a representative. The density value of the density measurement patch 9a (9a1, 9a2, 9a3, or 9a4) of each color on the printing product 9, which is extracted before or during final printing, is measured. The difference between the measured density value of each color and the preset reference density value of each color is obtained. The adjustment amount of the opening ratio of the ink key 4-1 (the adjustment amount of ink supply amount to the region S1) in each color printing unit is then obtained from the obtained density difference of each color. The resultant adjustment amounts (reference correction amounts) are multiplied by a unique coefficient (control ratio) to obtain a correction amount. The resultant correction amount is fed back to adjust the opening ratio of the ink key 4-1 in each color printing unit.
In a similar way, for regions S2 to Sn as well, the adjustment amounts of the opening ratios of the ink keys 4-2 to 4-n (the adjustment amounts of ink supply amounts to the regions S2 to Sn) in each color printing unit are obtained. The resultant adjustment amounts (reference correction amounts) are multiplied by a control ratio to obtain a correction amount. The resultant correction amount is fed back to adjust the opening ratios of the ink keys 4-2 to 4-n in each color printing unit.
Note that, in adjusting the opening ratios of the ink keys 4-1 to 4-n in each color printing unit, the only allowable density difference of each color is defined for the density difference (measured density difference) between the measured density value of each color and the preset reference density value of each color. The ink supply amount is adjusted for the corresponding ink key when the obtained measured density differences are larger than the allowable density difference (Japanese Patent Laid-Open No. 2003-118077).
In the above-described conventional ink supply amount adjustment method, an operator arbitrarily sets an allowable density difference for a measured density difference, for each color. When using ink, e.g., process ink of black, cyan, magenta, or yellow, which is generally used in printing, the operator can experimentally set the allowable density difference as an almost adequate value. However, when using ink (special ink) of a special color, which has never (rarely) been used, the operator does not obtain the adequate allowable density difference of each color.
Hence, when a wrong allowable density difference is set, in some cases, the ink supply amount is not corrected although it must be corrected. Alternatively, in some cases, the ink supply amount is corrected although it need not be corrected. This causes the hunting phenomenon (the color tone becomes unstable because of the variation in color density on the printing product) of the ink film thickness on the paper sheet, thus posing a problem. In addition, assume that the allowable density difference is small, and the ink supply amount adjustment interval (printing product sampling interval) is short. While the preceding adjustment of the ink supply amount is not sufficiently reflected on the printing product, the next ink supply amount adjustment is done. In this case, a hunting phenomenon of the ink thickness on the paper sheet also occurs.
In addition, the density value of a specific part of the printing product may temporarily largely vary due to an unexpected accident (paper fold error, paper breakage, or smear) on the printing product. In this case, when the ink supply amount is adjusted from the measured density difference as usual, the ink supply amount is adjusted to a value largely shifted from what the ink supply amount should be. As a result, the amount of wasted paper increases, thus posing a problem.